1. Field of the Invention
This invention relates to apparatus for and methods of reproducing data from a disk medium, capable of use in digital audio and similar systems.
2. Description of the Prior Art
Disk storage devices, such as those used for digital audio applications, typically employ one of two data storage techniques, namely constant angular velocity with constant read/write data rate or constant linear velocity with constant read/write data rate.
In the constant angular velocity with constant read/write data rate mode, the disk rotates at a constant rate, regardless of the radial position of the read/write head and the read/write data rate to the disk medium is constant.
One advantage of this mode is that the speed of the disk is independent of the position of the read/write head; thus there is no need to change the rotational speed of the disk when the read/write head changes position. This process of speeding up or slowing down the rotational speed of the disk can take longer than the positioning time of the head, and its avoidance decreases the latency of the disk system.
Another advantage is that, because the speed of the disk is independent of the position of the read/write head, there can be more than one independently positioned read/write head in simultaneous use. This technique can greatly increase the overall data rate of the disk system.
Also, since the data rate of the read/write head is constant, this allows the use of simple equalisers in replay electronics.
However, a disadvantage of this mode relates to the storage capacity of the disk. Referring to FIG. 1 of the accompanying drawings, there is shown a portion of a disk of known outside diameter D and centre C, used to record a signal of data rate B bits per second, where the maximum linear density of bits per unit length is L, where the density of tracks per unit length is T, and where the inside diameter of the recorded area RA is I. The following calculations provide the value of I which yields the maximum storage on the disk.
The length of the inside track is EQU .pi.I
The number of bits recorded on the inside track is EQU .pi.IL
The width of disk available for the recording of data is ##EQU1##
The number of tracks recorded is ##EQU2##
Since this is a constant angular velocity and constant data rate method, each track contains the same number of bits equal to EQU .pi.IL
Thus the number of bits which may be recorded on a disk is ##EQU3##
This may be rewritten as ##EQU4##
The second of these two terms contains only set values. Thus the maximum storage capacity corresponds to a maximum value of the first term EQU ID-I.sup.2
The first differential with respect to I is EQU D-2I
Giving a maximum at ##EQU5##
Inserting this value into the original equation the number of bits which may be recorded on a disk is ##EQU6##
In the constant linear velocity with constant read/write data rate mode, the disk rotates at a variable rate, depending upon the radial position of the read/write head and the read/write data rate to the disk medium is constant.
An advantage of this mode resides in the improved disk storage capacity as compared to the above-described constant angular velocity mode. Using the same constants as above and given a value of I, referring again to FIG. 1, the length of the inside track is EQU .pi.I
The number of bits recorded on the inside track is EQU .pi.IL
The length of the outside track is EQU .pi.D
The number of bits recorded on the outside track is EQU .pi.DL
The width of disk available for the recording of data is ##EQU7##
The number of tracks recorded is ##EQU8##
Since this is a constant linear velocity and constant data rate method, the "average" track contains the mean number of bits between the number on the inside track and that on the outside track.
Thus the number of bits which may be recorded on the average track is ##EQU9##
Thus the number of bits which may be recorded on the disk is ##EQU10##
This may be rewritten as ##EQU11##
Smaller values of I obviously lead to greater storage capacity; however zero is not a practical value for the inner radius and for fair comparison a value of D/2 will be used as for the previous case. This leads to a capacity of ##EQU12##
This is 50% greater than the capacity in the previous case as expressed in equation (1). Moreover, this can be increased still further by selecting a value of the linear diameter I less than D/2.
A disadvantage of this mode is that the speed of the disk varies with the position of the read/write head, the speed variation being proportional to the ratio between the outside and inside diameters, a ratio of 2 to 1 in the above case.
European Patent Application Serial No. 0 507 476 A discloses disk recording and reproducing apparatus in which reproduced data from a disk is supplied to a buffer memory in synchronism with a clock signal having a frequency dependent on the transfer speed of the reproduced data. During recording, the data is supplied to the buffer memory according to a constant frequency clock signal, and is read out of the memory for recording at a variable frequency clock rate.